INFLUENCE OF OXALIC-ACID ON EQUILIBRIUM AND KINETICS OF ISOTOPIC EXCHANGE BETWEEN PENTAVALENT AND HEXAVALENT NEPTUNIUM IN NITRIC-ACID SOLUTIONS

It has been demonstrated spectrophotometrically at 25-degrees-C and an ionic strength-mu = 1.0 M (KNO3 + HNO3) that the introduction of H2C2O4 into nitric acid solutions of NP(V) with [HNO3] = 0.1 to 1.0 M in the presence of nitrite ions results in displacement of the equilibrium between Np(V) and Np(VI) in favor of the accumulation of Np(VI). When [HNO3] > 1.0 M, the presence of H2C2O4 does not influence the position of the equilibrium. The values of the apparent equilibrium constant have been calculated for various concentrations of HNO3 and H2C2O4. It has been found that the isotopic exchange between Np(V) and Np(VI) proceeds considerably more slowly in oxalate solutions than in the absence of oxalate. The values of the rate constant of isotopic exchange calculated for 9-degrees-C, mu = 1.0 M (KNO3), [H2C2O4] = 0.01 M, and pH 2.2 and 3.5 are equal to 0.49.10(3) and 0.67.10(2) liter/mole.min, respectively. A mechanism of isotopic exchange, which includes electron transfer between oxalato complexes of Np(V) and Np(VI), have been proposed.